The invention is based on a gas-exchanging process for two-stroke internal combustion engines as defined hereinafter, and to a two-stroke internal combustion engine for executing this process.
It is known that the disadvantages of the gas-exchanging process when used with two-stroke internal combustion engines, whether with externally provided ignition or self-ignited, stem above all from the gas-exchanging process, wherein on the one hand a mixture that is both ignitable and combustible is to be present at the point of ignition at the time of ignition and, on the other hand, the proportion of waste gases remaining in the combustion chamber should be as small as possible during the scavenging process. Thus, it is known that 15-25 percent of the piston stroke cannot be used to perform useful work, because only the filling volume can be converted into work. For this reason the type of scavenging plays a decisive role, as do the supply and precompression of the combustion air before it is supplied to the combustion chamber by means of inlet ports valves. A poor scavenging process and disadvantageous mixture formation in the area of the point of ignition lead to misfiring, poor running, high hydrocarbon emissions and disadvantageous fuel consumption.
The most commonly used type of scavenging in two-stroke internal combustion engines is reverse scavenging. Although in this case the advantages of simple engine design and low production costs are in effect, it is especially difficult to obtain optimum scavenging ratios in the total rpm range, because the oscillating behavior of the waste gas system results directly from the port opening control present. Normally, the waste gas system is optimized in such a way that the advantageous scavenging ratios are present at high rpm. In the process the inlet ports are only opened relatively late after outlet ports, so that the low pressure wave has already ebbed, and the pressure wave reflected in the waste gas system arrives again at the outlet port before the port is closed. By means of this, extremely poor filling ratios result, with respect to combustion air and fuel to waste gas on the one hand and, on the other hand, a high residual content of waste gas results in the combustion chamber, which leads to poor operating characteristics. Although improved scavenging and filling ratios can be obtained by means of an additional, external supercharger and correspondingly high precompression of the combustion air, or by means of controlling engagement of the waste gas stream; however, the fundamental problem of reverse scavenging cannot be rectified. In contrast, head-scavenged two-stroke engines have the advantage that, by means of well-timed control of the intake and outlet valves, particularly in the area of the point of ignition inside the combustion chamber, very good scavenging is accomplished. However, the disadvantages of this are that, during the gas exchanges typical for two-stroke engines, correspondingly large control cross-sections of the valves (4-valve or multivalve systems) must be available to a valve control for good filling, or that a powerful charger (compressor) is required for the combustion air. Both the valve drive and the charger are labor- and cost-intensive. In addition, the power loss in head-scavenged, two-stroke engines is relatively high in comparison to reverse-scavenged two-stroke engines, which negatively affects fuel consumption, and NO.sub.x emissions in particular. Because of the short gas exchanges necessary, the valve drive must additionally operate with double frequency in comparison to the four-stroke engine, causing considerable problems for controlling the inertial forces and thus limiting the maximum rpm of the engine to a comparatively low value.
With a known gas-exchanging process (Peugeot, SAE-Paper 9008001 1990) of the generic type of the present invention, a relatively small quantity of combustion air is supplied in addition to the port-controlled reverse scavenging, via an intake valve in the cylinder head; this quantity of air does not serve in scavenging the quantities of gas present in the combustion chamber, but rather to process and transport the fuel supplied on the aspiration side. For this reason this quantity of compression air amounts at most to 10% of the cylinder filling. The crankcase as a pump acts in a way known per se as a compressor for the combustion air. The ineffective spaces resulting from the additional air conduits correspondingly reduce the degree of pumping effectiveness, leading to a worsening of the precompression of the combustion air, and hence to the aforementioned disadvantages.
With another known process of the generic type of the present invention a separate pump for compression air having a pressure level of 5-7 bar is used for the precompression of this separate compression air used for fuel transport or in the processing of the fuel/air quantity.
In both known processes, fuel injection must be interrupted when the pressure in the combustion chamber becomes higher than the pressure in the aspirating tube, in order to prevent the fuel-air mixture from being forced back. During this period of time between the blocking of the intake valve and the time of ignition, and particularly at a low charge, the fuel in the combustion chamber can mix with the charge supplied via the input port and output port and is greatly diluted, which is undesirable and can lead to problems during ignition. Moreover, the aforementioned disadvantages of the reverse scavenging process and the head-scavenging process remain, without an actual improvement in scavenging. With both processes, it is attempted to combine an injection on the aspiration side in the area of the cylinder head with a reverse scavenging process for a two-stroke engine.